Carbon black is a form of elemental carbon and it can be produced in varying particle sizes and structures. Structure is the degree of particle coalescence and agglomeration with high structure carbon blacks having more particles agglomerated into random structures than do low structure carbon blacks. Particle size and structure affect various qualities in rubber such as elongation, modulus, tear strength, tensile strength and resilience. Carbon blacks are used with both polar and nonpolar rubbers to enhance these qualities. Rubber stocks containing larger particle blacks display good resilience, but tend to be deficient in tensile strength and elongation. Conversely, rubber stocks containing small particle blacks display good tensile strength and elongation, but have poor resilience qualities. High structure carbon blacks improve the modulus and extrudability of rubber stock. The low structure carbon blacks improve the resilience characteristics of rubber stocks. The carbon blacks in the middle ranges provide stocks that have average characteristics. Manufacturing techniques for carbon blacks have been perfected so that large or small particle carbon blacks with either high or low structure can be prepared.
Carbon blacks are graded by ASTM designation D 1765 standard classification. There are at least four types of carbon blacks, including furnace blacks, lamp blacks, thermal blacks and channel blacks. Furnace blacks are not modified to alter their inherent properties on rubber cure rates. Lamp blacks or lamp black substitutes have modified surfaces that affect the cure rate of rubber. Thermal blacks include very large particle blacks. These blacks are manufactured by incomplete combustion of natural gas. Channel blacks are also produced by incomplete combustion of natural gas. The carbon particles which are intermediate in size, are deposited on steel channels over the flame. The surface of the particle is not altered to affect rubber cure rate. The structure of blacks is defined by the n-Dibutyl Phthalate Absorption number (DBP number) and is measured by the D 2414 test method. The larger the DBP number, the higher the structure. Particle size is defined by Iodine Adsorption Number and is measured by ASTM D 1510 test method. There is an inverse relationship between the iodine number and particle size; the higher the number the smaller the particle.
In manufacturing rubber products a carbon black with certain structure and particle size is used to enhance a certain desired characteristic while sacrificing others. For instance, a large particle black will be used to achieve desired compression characteristics at the expense of tear and modulus. Combinations of carbon blacks with the same size, but different structures have been used together. Using carbon blacks with the same size characteristics limits flexibility in varying qualities in the rubber products. However, utilizing carbon blacks with different sizes has generally been avoided, because mixtures of different particle sizes of carbon blacks are deemed incompatible.
A rubber products manufacturer uses different size and structure carbon blacks to produce goods with varying characteristics. Even so, utilizing one type of carbon black or pairs matched for size, limits the qualities achieved in the rubber, because of the enhancement of certain qualities is detrimental to others. Recommended formulations for transmission belting for engines, hydraulic hose tubes and wire and cable jackets include mixtures of large particle carbon blacks to provide desired compression characteristics. The Vanderbilt Rubber Handbook, R. T. Vanderbilt Co., Inc.; Norwalk, Conn.; 13th Ed. (1990) pp. 653, 721. Other formulations for high hardness rubber mix furnace blacks with large particle thermal blacks. Id. at 770. Typical formulations for seals include a matched pair of large particle carbon blacks with high and low structure. Id. at 737. Mixtures of carbon blacks with similar size and structure are used in some applications. For example, two large particle and low structure carbon blacks have been recommended for a tire body formulation. Id. at 606. Tire treads, on the other hand, contain small particle carbon blacks. Id. at 604, 605. The large particle and low structure is desired for resilience qualities in the tire body. The tread utilizes small particles with better tensile strength.
The present invention is a unique pairing of furnace carbon blacks with disparate qualities to provide a system that can be manipulated to produce rubber with balanced optimum qualities. Specifically, the carbon black pair system is a mixture of two furnace carbon blacks. The pairs are matched according to structure, but have different particle size. Typically, carbon black pairs are matched by size and the present invention is contrary to traditional teaching. One of the pair of carbon blacks has an Iodine Adsorption Number of greater than 115 and the other has an Iodine Adsorption Number of less than 115. The carbon black pairs have the same structure. For the invention, structure is defined as high structure for particles with a DBP number of greater than 110 and low structure for particles with a DBP number of less than 110. Therefore, both carbon blacks in the pair will have either high or low structure based on the 110 DBP number cutoff.
The two carbon blacks are used together with the ratio of one to another generally not to exceed 3:1. The ratio is varied according to the qualities desired. The use of the carbon black pairs of this invention mixed with rubber stock has produced an improved rubber product with good compression and resilience characteristics generally attributed to the use of large carbon black particles in a rubber while retaining the good elongation, modulus, tear and tensile strength attributable to the presence of small carbon black particles. The use of the unique carbon black pairs can produce a rubber composition with a combination of the optimum qualities of the large and small particles.
The carbon black pair system also includes, in combination with the carbon black pairs, a fumed silica and silane coupler. Fumed silica has a much smaller particle size than precipitated silica that is also used in rubber processing. Silica increases tear strength. Fumed silica is very expensive and builds viscosity in the rubber. A silane coupler is also used with the carbon black pair. Silane couplers used in conjunction with silica decrease elongation, but sometime can produce excessively high modulus elongation. Silane couplers are used in ranges known to those skilled in the art with the fumed silica. A typical ratio is 10:1 fumed silica to silane coupler. The fumed silica is used generally in a 12.5% to 50% by weight to the combined weight of the carbon black pair.
The carbon blacks can be selected according to the parameters of this invention to maximize the desired properties of polar and nonpolar rubbers. The use of the unique carbon black pairs with silica/silane system allows the manufacturer to maintain a minimal stock of carbon blacks and produce rubber products with a range of qualities. The unique carbon black pairs eliminate the need to maintain a wide range of different size and structure carbon blacks for manufacturing purposes.
The use of the carbon black pairs with a fumed silica and silane coupler produces unique rubber compositions. The carbon black system is mixed with the selected rubber stock and cured. The carbon black mixture comprises from about 40 parts per hundred to about 80 parts per hundred of the selected rubber. The rubber compositions, both polar and nonpolar rubber, have as the carbon black constituent with high or low structure carbon blacks with DBP numbers greater than and less than 110 respectively, and a mixture of large and small particle carbon blacks with Iodine Adsorption Numbers of less than and greater than 115. The use of the new carbon black pair produces a new rubber composition as described herein.
Carbon blacks are given an ASTM designation with the letter "N" used to indicate typical furnace blacks that receive no special surface modification to alter their influence on the rate of cure. The second character in the nomenclature is a digit to designate particle size with lower digits indicating smaller particles. The third and fourth characters are assigned arbitrarily, however the full designation of each of the carbon blacks assigned by the ASTM describes the carbon black as to particle size and the structure. For example, one of the pairs with different particle sizes and low structure is ASTM D1765 designations N231 a small particle (121 Iodine Adsorption No.) and low structure (DBP No. 92) and N774 a large particle (29 Iodine Adsorption No.) and low structure (DBP No. 72). By varying the amount of N231 and N774 within the range of 3 parts N231 to 1 part N774 or 1 part N231 to 3 parts N774, rubber stocks can be provided with varying characteristics in acceptable ranges for different rubber products. Another pair with different particle size and high structure is N234 a small particle (120 Iodine Adsorption No.) and high structure (DBP No. 125) and N550 a large particle (43 Iodine Adsorption No.) and high structure (DBP No. 121). Accordingly, the rubber products manufacturer can stock a limited number of carbon blacks and produce a variety of rubber stocks.